Clusters of spherical microparticles (SMP) that averaged 500 A in diameter and were composed of dense cores surrounded by single trilaminar membranes were found in operatively obtained biopsies from 29 of 70 patients with various types of heart diseases. SMP appear to form in heart as part of a process that mediates the remodeling of cellular surfaces, especially those of intercellular junctions undergoing dissociation. BIBLIOGRAPHIC REFERENCE: Ferrans, V.J., Thiedemann, K-U., Maron, B.J., Jones, Michael, and Roberts, W.C.: Spherical microparticles in human myocardium. An ultrastructural study. Lab Invest 35: 349-368, 1976.